FIG. 5 shows an example of a conventional fast reactor 1 disclosed in Japanese Patent Registration No. 3126524. As shown in FIG. 5, the conventional fast reactor 1 includes a reactor vessel 7 in which the fast reactor 1 is accommodated, and a reactor core 2 disposed in the reactor vessel 7 and loading thereon a fuel assembly. The reactor core 2 has generally a cylindrical shape. An outer circumference of the reactor core 2 is surrounded by a core barrel 3 that protects the reactor core 2. A reflector 4 is arranged outside the core barrel 3. The reflector 4 is connected to a reflector drive unit 12 through a drive shaft 11. The reflector 4 is capable of vertically moving around the reactor core by the reflector drive unit 12 so as to control a reactivity of the reactor core 2.
Disposed outside the reflector 4 is a partition wall 6 surrounding the reflector 4 and forming an inner wall of a channel for a coolant 5. The channel for the coolant 5 is formed in a gap between the reactor vessel 7 and the partition wall 6. In the channel for the coolant 5 between the reactor vessel 7 and the partition wall 6, there is disposed a neutron shielding member 8 that surrounds the reactor core 2. The reactor core 2, the core barrel 3, the partition wall 6, and the neutron shielding member 8 are all placed on and supported by a reactor-core support plate 13.
FIG. 6 shows an example of the structure of the reflector 4 disclosed in Japanese Patent Registration No. 3126502. The reflector 4 includes a neutron reflecting part 4a and a cavity part 4b integrally disposed on an upper portion of the neutron reflecting part 4a. The cavity part 4b is formed of a housing. An inside of the housing is filled with a gas 41 whose neutron reflecting ability is inferior to that of a coolant 5, or is maintained in a vacuum condition. Owing to the cavity part 4b, a reactivity can be suppressed, as compared with a case in which an outside of a core barrel 3 is covered with the coolant 5. Thus, enrichment of fuel can be increased, whereby a reactivity life of a reactor core 2 can be elongated. A reflector drive unit 12 is connected to an upper portion of the neutron reflecting part 4a through a drive shaft 11. In FIG. 6, the same parts as those in FIG. 5 are shown by the same reference numbers.
A temperature of the coolant 5 in the fast reactor 1 is between about 350° C. and about 500° C. To be specific, the temperature of the coolant 5 is about 500° C. near the reactor core 2 inside the core barrel 3, and is about 350° C. near the neutron shielding member 8 outside the partition wall 6. Namely, the temperature of the coolant 5 near the core barrel 3 and the temperature of the coolant 5 near the partition wall 6 vary by about 150° C. In addition, since the coolant 5 is heated from about 350° C. to about 500° C, the temperature of the coolant 5 inside the core barrel 3 axially varies by about 150° C.
In the neutron reflecting part 4a and the cavity part 4b of the reflector 4, since the temperature significantly varies in both the radial and the axial directions, the reflector 4 may be deformed by a thermal expansion difference. The deformed reflector 4 may come into contact with the core barrel 3 and/or the partition wall 6, when the reflector 4 is dropped down in order to urgently shut down the fast reactor 1. In this case, the reflector 4 may fail to fall down within a predetermined period of time.
In addition, because of the temperature difference of the reflector 4, there is a possibility that a thermal stress and/or a creep generate in the reflector 4 to damage the neutron reflecting part 4a and/or the cavity part 4b of the reflector 4.